1. Field of the Invention
The present invention relates to a portable radio communication device such as a radio transceiver.
2. Description of the Background Art
Conventionally, a whip antenna has usually been employed for a portable radio communication device such as a radio transceiver. However, as the whip antenna extends from the device body, it is often damaged during operation.
There is an alternative antenna configuration free of the drawbacks of the whip antenna, in which a line-shaped element is attached at the top of a monopole antenna such that the antenna height can be reduced. Examples of this type of antenna include the L-type antenna and the inverted F-type antenna. However, this type of antenna has been known to have a narrow bandwidth.
In order to resolve this problem, there has been a type of antenna in which the line-shaped element is replaced by a plate shaped element. Examples of this type of antenna includes the plate-shaped inverted F-type antenna. However, in this type of antenna, the bandwidth becomes progressively narrower as the size is made progressively smaller.
In view of this, there has been a proposition of an S-type antenna in which the plate-shaped element is used in the line-shaped T-type antenna. In this S-type antenna, the element attached at the top of the monopole antenna has a spiral shape that can be obtained by combining two of the inverted F-type antennas or the L-type antennas at the feeding point, and this configuration is considered to be one of the factors contributing to the wider bandwidth of the S-type antenna.
However, using an S-type antenna with a very low antenna height can be associated with the following problems.
Firstly, in this type of an antenna having a capacity load attached at its top, the current at a shortened monopole antenna portion can be considered as a component contributing to radiation in general. Consequently, when the antenna is made to have a very low antenna height, it is expected that the total current on the monopole antenna portion reduced and the radiation resistance is consequently reduced such that the Q-value of the antenna at a time of resonance becomes higher while the bandwidth characteristic of the antenna becomes narrower.
Additionally, in an antenna generally used for a radio communication device, a contribution to a radiation comes not just from the current on the antenna but also from the current on the device body. Consequently, in the above-described line-shaped antenna, the antenna element and the device body usually form a type of dipole antenna which creates radiation. This situation additionally applies to the L-type antenna and the T-type antennas.
In the T-type antenna, the element attached at the top of the monopole antenna portion can be regarded simply as a capacitive element, but it is also possible to regard this element itself as an antenna formed by combining two L-type antennas when the T-type antenna is made to have a very low antenna height, such as a height less than .lambda./50, where .lambda. is a wavelength of a radio signal used in radio communication. Consequently, when the physical length from a connection point of the monopole antenna portion and the line-shaped element to a tip end of the line-shaped element is different for two ends of the line shaped element as the connection point is displaced from a middle, this antenna has two different resonant frequencies, such that a dual resonance occurs. In this case, this antenna shows a radiation characteristic of an L-type antenna at each of these two resonant frequencies. Here, the difference between these two resonant frequencies is small when the Q-values at the two resonant frequencies are high, and becomes progressively larger as the antenna height is progressively lowered.
Conversely, the line-shaped element attached at a top of the monopole antenna functions only as a capacitive element when the antenna height is of an order of .lambda./4 or .lambda./5, but as the antenna height is progressively lowered, it begins to behave as a line with a length of .lambda./2. The monopole antenna portion then appears to function as a feeder for feeding currents to this line. As a result, the line-shaped element itself starts to resonate as a resonator for .lambda./2. In such a .lambda./2 resonance mode, the currents flowing on the device body are expected to cancel out each other. Consequently, the radiation from the device body is reduced and the radiation resistance of the device as a whole is reduced. However, the currents flow on the line-shaped element and the device body below the line-shaped element, Such that conductor loss is still present.
Consequently, even when impedance matching between the feeder and the antenna is seemingly established, there are cases in which the current component for the conductor loss becomes larger than the current component for the actual radiation. Here, the conductor loss becomes unignorable when the antenna height is very low and the radiation resistance is low, as well as when the resonance in the radiation mode is not occurring at this frequency.
Thus, in general, in T-type antenna with a low antenna height, the resonant frequency of the resonator for .lambda./2 and the resonant frequencies of the dual resonance due to the displaced connection point are very close to each other. Consequently, when the two resonant frequencies of the dual resonance due to the displaced connection point are relatively separated compared with the bandwidth of the line-shaped element, the resonance at .lambda./2 on the line-shaped element and the radiation from this element become predominant at the frequencies between these two resonant frequencies of the dual resonance, and the antenna efficiency is reduced for such frequencies.
According to these considerations, it can be asserted that, in the T-type antenna, the lowering of the antenna performance can be prevented by making the physical length between feeding point and tip end of the line-shaped element to be identical for two ends of the line-shaped element.
However, in practice, even when the antenna is attached to the device body with such an adjustment of the physical lengths in the antenna, the electrical length between the feeding point and the tip end of the line shaped element is not necessarily identical for two ends of the line-shaped element, because of the asymmetrical shape of the device body with respect to a point at which the antenna is attached and the asymmetrical arrangement of the other circuit components provided in the device body.
In general, an equivalent circuit for the tip end of the antenna is expressed by a capacitive element whose capacitive characteristic represents that between the tip end of the antenna and the ground plate to which the antenna is attached. However, in an antenna made in a low antenna height by attaching the line-shaped element having a plurality of tip end portions as described above, it is considered that the capacitive characteristic of each tip end portion is not uniform with respect to the entire ground plate, and produced by the strong coupling of each tip end portion with the ground plate in a vicinity of that tip end portion.
Consequently, depending on the state of the ground plate in a vicinity of each tip end portion, the capacitive characteristic of each tip end portion varies, and the difference in capacitive characteristics for different tip end portions can cause the difference in the electrical length between the feeding point and the tip end for the different tip end portions, which in turn causes the lowering of radiation efficiency due to the occurrence of the dual resonance, as described above.
In addition, in a device using this type of antenna, the device body also functions as a ground plate and the currents on the device body also contributes to the radiation. In this regard, in order to reduce the influence on the antenna characteristic due to the hand of the user holding the device body, it is necessary to mount the antenna on an upper portion of the device body, so that the antenna and the device body are inevitably arranged asymmetrically, and this asymmetrical arrangement of the antenna and the device body causes the deterioration of radiation efficiency for the reason already described above, even when the line-shaped element itself is formed symmetrically with respect to the feeding point.
In view of the convenience for carrying, it is preferable for the portable radio communication device to be of a small size. However, when the radio communication device is made in a small size, the hand and the head of the user holding the device body are going to come into an even closer proximity to the antenna such that the radiation field of the antenna can be affected.
As a scheme to reduce this interaction between the antenna and the user's body, it is possible to provide a measure for not directing the radiation toward the user's body. To this end, it is necessary for the antenna to have a definite radiation directivity. As an example of such an antenna with a simple configuration, there is a configuration in which passive elements are arranged in an array around the antenna such that any desired radiation directivity can be obtained by appropriately setting the arrangement of the antenna and the passive elements.
However, since the human head also possesses some conductive property for radio frequency electromagnetic waves, when the antenna is mounted on the top end of the device body, the electrical projection image of the antenna is formed on the surface of the user's head. The radiation field obtained by appropriately adjusting the antenna element as described above is then severely affected by this projection image on the user's head, and there are cases in which the desired radiation directivity cannot be realized.
Even when the radiation field is not directed toward the user's body, there is a problem of a direct interaction between the hand and the head of the user with currents on the device body contacting the hand and the head of the user. In particular, in a case where the antenna is a .lambda./4 monopole antenna or a built-in antenna such as a plate shaped inverted F-type antenna, the radio frequency currents flowing on the device body are relatively large and make a relatively large contribution to the radiation field of the entire radio communication device. The influence due to the human body in this case is going to be unignorable.
As a scheme for resolving this problem, it is possible to consider separating the device body part contacted by the user's body from the device body part connected with the antenna at radio frequencies. However, for a small sized radio communication device, these separated parts are inevitably located fairly close to each other, so that it is rather difficult to disconnect these separated parts completely, unless the size of the radio communication device itself is increased considerably to incorporate the mechanism necessary to achieve such a complete disconnection between these separated parts.
In this regard, as an antenna suitable for a portable radio communication device, there has been a proposition for a sleeve monopole antenna with a balun as shown in FIG. 1, which shows a radio communication device comprising a device body 210 equipped with a speaker 211, a display 212, a keyboard 213, and a microphone 215, and an antenna 216 equipped with a cylindrical conductor body 217a with a length of about .lambda./4 called a balun for providing a choking effect with respect to the radio frequency currents. Because of this balun 217a, this sleeve monopole antenna of FIG. 1 has a characteristic of not allowing the radio frequency currents to flow on the device body 210 compared with an antenna without a balun, so that the reduction of the influence due to the user's body can be expected. However, in order to actually construct this antenna of FIG. 1, it is necessary to cover the coaxial feeder with a cylindrical dielectric body 217b, and then providing the balun 217a over this dielectric body 217b, so that the structure of the antenna 216 becomes quite complicated. As a result, it becomes difficult to provide any resiliency to this antenna 216 itself, and therefore this antenna 216 can be easily damaged while carrying, and it is difficult to make this antenna in a small size.
On the other hand, in order to simplify the configuration of such a sleeve monopole antenna, if the balun 217a is removed, the resulting radiation field appears as indicated in FIG. 2, which shows the radiation field for an antenna shown in FIG. 3 comprising a feeder 219 of .lambda./4 length connected with an antenna element 218 of .lambda./4 length mounted on the device body 220 made of a conductive body. In terms of their lengths, the feeder 219 and the antenna element 218 of this antenna of FIG. 3 appear to constitute a dipole antenna having a radiation field in a shape indicated in FIG. 4, but the actual radiation field of this antenna of FIG. 3 indicated in FIG. 2 largely differs from this ideal radiation field of the dipole antenna indicated in FIG. 4. This difference is caused by the currents flowing over the outer conductive portion of the feeder 219 from a feeding point located at a connection point between the antenna element 218 and the feeder 219, which are not stopped at the connection point between the feeder 219 and the device body 220. These are further flown into the device body 220, such that the radiation from the currents flowing over the device body 220 affects the radiation field of the antenna as a whole. The radiation field actually shown in FIG. 2 is obtained for the device body 22 with a body length of about .lambda., so that the the radiation field indicated in FIG. 2 rather resembles the ideal radiation field for the dipole antenna for (2/3).lambda. as indicated in FIG. 5.
Thus, by removing the balun, the radiation field of the radio communication device is largely changed from that of the ideal .lambda./2 dipole antenna indicated in FIG. 4 to that if FIG. 2, while the influence due to the interaction between antenna and the user's hand and head and the device body contacted by the user's hand and head cannot be prevented because of the radio frequency currents flowing into the device body.